Direct response to divergent selection for yearling growth rate in Angus cattle

ELSEVIER

Livestock Production Science 49 (1997) 297-304

Direct response to divergent selection for yearling growth rate in Angus cattle P.F. Parnell ‘, P.F. Arthur Agricultural

*, R.

Research Centre. New South Wales Agriculture,

Barlow ’

Trangie, N.S. W. 2823 Australia

Accepted 17 March 1997

Abstract An experiment to evaluate the effects of selection for yearling growth rate (average daily gain from birth to yearling age) on the major components of beef herd profitability was started in 1974. This paper describes the design, population structure, selection applied and the realised direct response to selection. Two divergent selection lines (high line and low line) and an unselected control line were created in a closed Angus herd. Generation intervals after 17 years of selection were 3.2 yr, 3.3 yr and 3.3 yr for high, control and low lines, respectively, resulting in a corresponding 5.5, 5.0 and 5.1 generations of selection. The high effective population sizes of 67, 106 and 71 per generation for high, control and low selection lines, respectively, resulted in a low rate of inbreeding per generation for each selection line (0.74% for high line, 0.53% for control line and 0.88% for low line). Average selection differentials per year were 0.016 kg/day and - 0.018 kg/day for the high and low selection lines, respectively, resulting in a corresponding direct selection response per year of 0.006 kg/day and -0.007 kg/day. Using animals recorded prior to 1964 as the base, the mean estimated breeding value (EBV) for yearling growth rate at the start of the experiment (1974) was calculated to be 0.015 kg/day. After 17 years (1991) of selection average yearling growth rate EBV for high line calves was 0.115 kg/day, compared to 0.030 kg/day for the control line and -0.060 kg/day for the low line calves. Realised he&abilities were 0.37 + 0.09 for the high line and 0.38 + 0.09 for the low line. 0 1997 Elsevier Science B.V. Ke.vwords: Beef cattle; Selection response;

Yearling gain; Realised heritability

1. Introduction In the early 1970s beef producers began to place considerable emphasis

rate and size. Growth rate was relatively easy to measure, and was known to be closely related to the commercial value of individual animals, especially when cattle are grown in feedlot (Dickerson et al., 1974). While it was known that selection for increased growth rate would result in faster growing animals which were heavier at all ages, there was little information available about the expected changes in other components of herd profitability. In addition, any adverse relationships between growth performance and other commercially important traits such as reproductive performance, for growth

in Australia on selection

* Corresponding author. Tel.: +61 68 887404: fax: +61 68 887201; e-mail: [email protected] ’ Present address: Angus Society of Australia, Armidale, N.S.W. 2350, Australia. ’ Present address: NSW Agriculture, Orange, NSW 2800. Australia. 0301-6226/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO3OL6226(97)00045-6

298

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Production Science 49 (1997) 297-304

milk production, feed conversion efficiency and body composition could further offset any gains achieved by improving growth potential. These concerns were not only limited to Australia, but to beef producers world-wide. Results based on selection in cattle were needed rather than theory alone or results from selection in laboratory animals. To complement the cattle selection experiments already in progress (listed in the review by Mrode, 19881, selection experiments were set up in Australia and New Zealand (listed in the review by Pamell and Morris, 1994) in the 1970s to address some of these issues under pasture conditions. The objective of this experiment was to evaluate the effects of selection for yearling growth rate on each of the major components of beef herd profitability in temperate Australia. This included the investigation of responses in growth and body size; reproductive performance and maternal ability, herd feed requirements and body composition of steers. This paper provides a description of the design of the experiment, data collection procedures, population structure, nature of the selection applied, and realised direct response to selection for yearling growth rate.

2. Materials and methods 2.1. Location and environment The experiment was conducted at the Trangie Agricultural Research Centre, (31”15O’S, 147”57’E), located on the central western plains of New South Wales (NSW), Australia. The average long term annual rainfall at the research centre is 480 mm, and is typically nonseasonal and variable. Perennial pastures included windmill grass (Chlotis truncata), spear grass (Stipa spp.) and wallaby grass (Danthonia sp.). Annuals were primarily barley grass (Hordeurn leporinum), rats-tail fescue (Vulpia myuros), burr-medic (Medicargo spp. > and crowsfoot (Erodium sp.). Much of the summer feed consisted of dry residue from winter annuals as well as burr-medic. 2.2. Foundation population Cattle used in this experiment were from an earlier project (1963-1973) designed to demonstrate the

application of selection procedures based on sound genetic principles, and to provide superior genetic material for use in the Australian beef industry. In 1963, the breeding herd was made up of registered Angus cattle (120 females and 8 bulls) maintained at the Research Centre. In 1964, 3 bulls from commercial herds were purchased and used, to widen the gene pool of the research herd, after which no further outside genetic material was introduced. During the 1963-1973 period, selection was based on an index of adjusted yearling weight and conformation score. 2.3. Formation of growth rate selection lines The experiment commenced in 1974 with the establishment of three closed selection lines. Fig. 1 illustrates the establishment of the selection lines in 1974-75. Of the 220 performance recorded cows in the Trangie Angus herd at the time, a group of 50 were randomly chosen to form a control line. Of the remaining cows, 85 were allocated to the high growth rate line (high line) and 85 to the low growth rate line (low line), based on their individual yearling growth performance. Of the 50 yearling bulls available, 10 were randomly chosen to be used in the control line. Seven of the remaining bulls were allocated to the high line and 5 to the low line, based on their individual yearling growth performance. This design was chosen to provide a rapid divergence in growth rate between the high and low selection lines, with the control line providing a base for the measurement of selection responses. It was anticipated that the divergent design would generate differences between the high and low lines over a 15 year period

Original

Angus

Herd

I Selection ppwtb

selected Y-w (nigh

for hi * line)

1

based

on

Random

rate

Selected

for low

Y-h gpia (Law line)

Selection

No selection

for

yearling

gain

(Control

line)

Fig. 1. Development of the three growth rate selection lines.

P.F. Pamell et al. /Livestock Production Science 49 11997) 297-304 Table I Multiplicative

adjustment

Age of dam

2 years 3 years 4 years and older

factors for yearling

gain

Sex of calf male

female

1.07 1.02

1.05 1.01 1.00

1.00

that would approximate the responses achieved over a 30 to 40 year period in a conventional unidirectional selection program. The sole selection criterion for all replacement bulls and heifers in the high line and low line was individual growth rate from birth to yearling age (yearling gain), adjusted for age of dam effects. All replacement bulls and heifers for the control line were strictly chosen at random. The multiplicative adjustment factors used for the calculation of adjusted yearling gains for high line and low line calves are presented in Table 1. These values were obtained from the analysis of data collected in tbe Trangie herd and several other New South Wales Angus herds prior to 1973. Only animals with gross structural problems or severe illness were excluded as candidates for selection. 2.4. Herd structure and management From 1974 to 1982 the high and low lines were each maintained with approximately 85 breeding females and 5 sires used per year. The control line had approximately 50 breeding females and 10 sires used per year. During the period from 1977 to 1982 an attempt was made to optimise the age structures among breeding cows in the high and low lines to maxim&e the genetic gains in each subsequent generation. A computer program developed by Hopkins and James (pers. corn.) was used to determine the ‘optimum’ proportions of cows to be selected from each age group in the high and low lines. This program accounted for the effect of overlapping generations on the expected genetic merit of each age group (Hopkins and James, 1977). During 1983 to 1988 the herd was expanded in size by retaining all potential breeding females to

299

enable the establishment of satellite herds at two other locations. The retention of all available breeding females effectively removed any female selection differential, thereby reducing the expected rate of selection response. During 1986 to 1988 bull selection differentials were also reduced due to the random selection of male calves from each line for use in steer feed conversion efficiency and body composition studies. Throughout the project replacement bulls and heifers were mated at 14 months of age, and bulls were used for only one breeding season. Cows were culled if they failed to calve in two consecutive years, or reached 7 years of age. Animals from each line were grazed together throughout the year, except during mating. Allocation of mates within selection line was completely at random, except for the avoidance of half-sib and son-dam matings. In all years, heifers were placed with selected bulls in mating paddocks at about 14 months of age and remained in these mating groups for a total of 12 weeks. The mating season of the main breeding cow herd was a 9 week period, which started 3 weeks after the heifer mating season. Following calving, cows with bull calves were grazed as one management group and cows with heifer calves grazed as another management group. Following weaning at approximately 200 days of age, calves were grazed within sex groups on irrigated pasture which was predominantly luceme ( Medicago satiua), phalaris (Phalaris tuberosa), white clover (TrijXum repens) and fescue (Festuca spp). Supplementary feed (chopped luceme hay and oats) was offered to calves during the postweaning period in years of limited pasture growth. 2.5. Measurements

taken

A comprehensive performance recording program provided a large database of records for the analysis of responses over time in each of the selection lines. Calves were tagged and measurements of body weight recorded at birth. Calves were subsequently weighed approximately every 6 weeks until yearling age (about 365 days). The first calf weight followed a morning fast and all other weights followed an overnight fast.

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P.F. Pamell et al. /Livestock Production Science 49 (1997) 297-304

2.6. Genetic ana! statistical

analyses

2.6.1. Generations and selection diferentials The generations of selection applied were calcu-

lated as GC = 1 + (GC, + GC,)/2 (Brinks et al., 19611, where GC is the generation coefficient of the calf, and subscripts s and d refer to sires and dams, respectively. Foundation animals were assigned a GC of zero. Generation interval was calculated as the average age of the parents when their progeny were born, being the inverse of the regression of GC on year of birth of calf (1974-1991). Annual selection differentials were calculated from difference between the average performance of those animals selected as parents each year and the average performance of all animals in each respective line. 2.6.2. Effective population

size and rate of inbreed-

ing

The effective population size N, for each selection line during each year of the experiment, was calculated using the formula: N, = 4 N, NJ( N, + N,), where N, was the number of male parents and Nr was the number of female parents represented in each annual calf drop (Falconer, 1989). The annual rate of inbreeding for each selection line was determined from the average inbreeding coefficients among progeny, as computed from the diagonal element of tbe inverse numerator relationship matrix for all animals (Henderson, 1976). 2.6.3. Realised direct response and genetic trend Annual realised responses for adjusted yearling growth rate in the high and low lines were measured as the difference between the average performance of animals in each line and the average of contemporary animals in the unselected control line. Average performance was computed using least squares procedures and fitting the fixed effects of selection line, year of birth, age of dam, sex of calf and age of calf. Since the management of animals in each line was identical, any observed differences in the mean performance of the lines could be attributed to genetic selection response. Average annual genetic response (genetic trend) was estimated for each selection line by linear regression of the annual direct responses (deviation

from control line) on year of birth of calf (19741991). The selection lines originated from the same base population, therefore the regression was constrained to pass through the origin, as suggested by Hill (1972) and confirmed by Baker et al. (1991). Thus the genetic response estimated includes a component of the preliminary response and an annual trend. 2.6.4. BLUP estimates Annual trends in yearling gain (kg/day) Estimated Breeding Values (EBVS) were computed by fitting all pedigree and yearling gain data to a best linear unbiased prediction (BLUP) reduced animal model, using genetic parameters obtained from the Trangie herd (heritability of 0.38; genetic variance of 2.86) and fitting nongenetic effects for year of birth, sex of calf, age of dam, calf age and inbreeding coefficient. Pedigree information recorded prior to 1963 and performance records collected from 1963 to 1974 were also included in the analysis. Animals recorded prior to 1964 were used as the base for the calculation of EBVs. 2.6.5. Realised heritability The realised heritability for adjusted yearling growth rate was determined for both the high and low lines separately by regressing the annual selection responses against the cumulative selection differentials, with the regression constrained to pass through the origin. Approximate standard errors for the realised heritability estimates were calculated accounting for drift variance and measurement error variance (Hill, 1971).

3. Results and discussion The population structure of the selection lines during the experiment is presented in Table 2. Among the selection lines, means for effective population size and generation interval were similar. The relatively large effective population sizes ensured low annual rates of inbreeding in each of the lines (average of 0.14% per year), minimising the impact of inbreeding depression on the rates of selection response. This is in contrast with tbe relatively high inbreeding rates obtained in earlier growth selection

301

P.F. Pamell et al. /Livestock Production Science 49 (1997) 297-304 Table 2 Population Year

1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 Mean

structure for high, control and low yearling High line

growth rate selection lines

Control line

effective population size

generation

interval

male

female

mean

25 18 22 18 18 18 18 18 18 22 26 22 21 22 26 22 22 21.0

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 3.0 3.0 2.0 2.0 2.1

4.4 4.1 3.8 4.2 4.1 3.8 4.1 3.7 4.0 4.1 4.8 4.9 5.0 4.7 5.0 4.2 4.2 4.3

3.2 3.1 2.9 3.1 3.0 2.9 3.1 2.9 3.0 3.1 3.4 3.5 3.5 3.9 4.0 3.1 3.1 3.2

Low line

effective population size

generation male

female

mean

33 30 32 34 33 31 32 36 37 37 34 34 31 32 39 32 22 32.3

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 3.0 3.0 2.0 2.0 2.1

4.4 4.2 3.9 4.7 4.4 4.1 3.9 3.9 4.3 4.6 5.2 5.5 4.9 5.2 5.3 4.1 4.4 4.5

3.2 3.1 2.9 3.3 3.2 3.0 2.9 3.0 3.2 3.3 3.6 3.8 3.4 4.1 4.2 3.1 3.2 3.3

experiments in beef cattle (Brinks et al., 1965; Irgang et al., 1985a,b; Nwakalor et al., 1986). Inbreeding rates obtained in the New Zealand growth selection experiment (Baker et al., 1991) were low and similar to those of this study. Least squares means for annual yearling gain of progeny for each of the selection lines is presented in Table 3. The actual response measured in any particular year fluctuated in each line due to genetic sampling and to available feed supplies. However the overall trend showed a divergence of the high and low selection lines from the unselected control line. A summary of the population parameters, selection differentials and direct response for each selection line, over all the years of the study, is presented in Table 4. The deviations of the high and low selection lines from the control line are not independent since they both involved the same control mean. Thus the divergence between the high and low selection lines represents the expected response after more than 10 generations of unidirectional selection for yearling gain. The responses per year of 0.0057 kg/day for

interval

effective population size

generation male

female

mean

18 22 22 22 18 18 18 18 18 22 25 32 21 22 26 22 22 21.6

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 3.0 3.0 2.0 2.0 2.1

4.3 4.2 3.8 4.5 4.0 3.7 3.7 3.8 4.4 4.4 4.8 5.2 5.4 5.1 5.1 4.5 4.5 4.5

3.1 3.1 2.9 3.2 3.0 2.8 2.9 2.9 3.2 3.2 3.4 3.6 3.1 4.1 4.4 3.2 3.2 3.3

interval

Table 3 Total number of records analysed, and least squares means and standard errors for yearling gain (kg/day) for each year born group of calves in the high, control and low selection lines a Year

High line

Control line

Low line

Number 1975 1976

1429 0.794*0.015 0.749*0.013

1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

0.709*0.013 0.725 f 0.012 0.653 *0.012 0.728 kO.013 0.800~0.014 0.722f0.016 0.727 f 0.010 0.754+0.010 0.747 + 0.011 0.73 1+ 0.009 0.762+0.016 0.673+0.012 0.600 f 0.008 0.583 f0.009 0.779 f 0.011

926 0.754*0.018 0.732ItO.019 0.632 kO.021 0.665 kO.017 0.613f0.015 0.669-10.015 0.701 kO.016 0.688 f 0.019 0.662 kO.016 0.663*0.014 0.674i-0.014 0.646+0.013 0.707 f 0.022 0.626+0.013 0.548 + 0.009 0.527 f 0.012 0.667 + 0.011

1244 0.728 rf: 0.016 0.737*0.013 0.591 *to.015 0.652f0.011 0.572&-0.014 0.593 kO.014 0.635 +0.013 0.601 iO.019 0.562 f 0.010 0.598 +O.Ol 1 0.563 + 0.012 0.564*0.012 0.608 * 0.024 0.501 f 0.012 0.458 + 0.009 0.477 + 0.013 0.580*0.014

a Means for 1994 (base population)

was 0.662 + 0.01

I kg/day.

P. F. Pamell et al. / Livesrock Production Science 49 (1997) 297-304

302 Table 4 Summary

of population

parameters,

selection differentials

and responses

in yearling

gain for high, control and low selection lines

Item

High line

Control line

Low line

Generations of selection Generation interval (years) Number of bulls mated per year Number of cows mated per year Effective population size per generation Average inbreeding rate (46) per generation Average selection differential per year (kg/day) per year (8) per year (phenotypic s.d.) per generation (phenotypic s.d.) Average selection response per year (kg/day) per year (%I per year (phenotypic s.d.) per generation (phenotypic s.d.) Realised heritability

5.5 3.2 5-6 85-120 67 0.74

5.0 3.3 10-12 50-100 106 0.53

5.1 3.3 5-6 85-120 71 0.88

s.d. represents

0.016 2.71 0.207 0.641

- 0.018 -3.15 - 0.240 - 0.804

0.006 0.87 0.075 0.253 0.37 f 0.09

- 0.007 - 1.05 -0.091 - 0.304 0.38 f 0.09

standard deviation.

the high line and -0.0070 kg/day for the low line are equivalent to 2.11 kg (0.0057 kg/day X 365 days) in liveweight at yearling age for the high line and -2.54 kg in liveweight at yearling age for the low line. The annual selection responses and cumulative selection differentials are presented in Fig. 2. The average selection response observed was greater in the low line than in the high Line. This was largely due to the greater selection differential achieved in the low line relative to the high line. Selection for growth in beef cattle has been based on either weaning weight, yearling weight, preweaning gain, postweaning gain or combinations of these with or without other traits. Most of the beef cattle Highline 0.15 0.10

II* = 0.37

I

I

A

-0.15 4 -0.30

I

-0.20

410

Cummtitie

0.00 mkction

0.10

030

030

differential @g/day)

Fig. 2. Selection differentials and responses high and low selection lines.

in yearling gain in the

selection studies on growth were included in the reviews by Mrode (1988) and Pamell and Morris (1994). Yearling gain, used in this study, has not been used in the other studies and it captures both preweaning and post weaning gain. Yearling gain was measured as kg/day, thus when multiplied by 365 days, it is equal to yearling weight, less birth weight. There is a near perfect phenotypic (r = 0.99) and genetic (r = 0.99) correlation between yearling gain and yearling weight (Koots et al., 1994b). Thus comparisons of the results of this study can be made with those studies where selection was based on yearling weight. In this study there were more generations of selection, shorter generation intervals, higher selection differentials and larger effective population sizes in the high line and low line compared with most other beef cattle selection studies based on any of the measures of growth. The direct selection responses in the high and low selection lines of this study were slightly higher than the average of about 0.8% per year achieved in other studies for yearling weight selection (Mrode, 19881, but lower than the estimate of 1.44% predicted by Smith (1984) for yearling weight in a large theoretical population. This discrepancy can largely be attributed to the shorter generation interval assumed by smith (1984).

P. F. PameN et al. /Livestock Production Science 49 (1997) 297-304 Eigb line 0.12

This study has demonstrated the effectiveness of selection as a tool to genetically improve growth in beef cattle. By using yearling bulls for mating the generation interval was reduced, thus increasing the annual rate of genetic gain. Genetic parameters obtained in this study are already being used in the Australian National Beef Recording Scheme’s genetic evaluation system, BREEDPLAN (Schneeberger et al., 1991).

T

1

0.08 -.

$

0.04 --

3 .i

A

l

.I

II 0.00 -.

P 5 z

A

Control line l

’

.

:

:

.

.

I

:

.

.

I

:

.

.-

:

:

-.

.

l

.

.

-0.04 _

-0.08 --

-0.124 74

:

: 76

i

: 78

:

So

I 82

:

84

86

88

: 90

:

303

I

92

Birtbyur(19-)

Fig. 3. Trends in yearling

gain estimated breeding values (EBVS).

Fig. 3 shows the trends in average estimated breeding values (EBVS) for yearling gain in each of the selection lines. The differences in average yearling gain EBVs over time generally reflected the trend in realised differences between the selection lines. Mean EBV for yearling gain at the start of the experiment (1974) was 0.015 kg/day. After 17 years (199 1) of selection average yearling gain EBV for high line calves was 0.115 kg/day, compared to 0.030 kg/day for the control line and -0.060 kg/ day for the low line calves. Realised heritability was similar for the high and low selection lines (Fig. 2). The values are consistent with those obtained for yearling weight selection experiments in beef cattle, as reviewed by Mrode (1988), and to those obtained in the yearling weight selection experiment in New Zealand (Baker et al., 1991) and in the USA (Koch et al., 1994). The values are similar to the weighted mean direct heritability for yearling gain of 0.34 summarised from 23 studies in beef cattle by Koots et al. (1994a). The values are also similar to the average heritability of 0.39 for yearling weight reported by Woldehawariate et al. (1977) from a summary of literature estimates based on analyses of covariance between relatives. They correspond with paternal half-sib heritability estimates for yearling weight reported by Carter et al. (1990) for New Zealand data and REML animal model estimate of the Trangie Angus data reported by Meyer (1994). The mean direct heritability estimates for yearling weight from recent reviews in beef cattle were 0.31 (Mohiuddin, 1993) and 0.33 (Koots et al., 1994a).

Acknowledgements Financial support was provided in part by the Meat Research Corporation of Australia (formerly Australian Meat and Livestock Research and Development Corporation). The contribution of I. Hopkins and J.W. James towards the design of the study is gratefully appreciated. The contribution of present and former staff of NSW Agriculture is acknowledged, especially C. Barnes, B. Bootle, A. Bums, C. Brennan, S. Chambers, R. Cox, P. Dunbar, S. Exton, R. Freer, B. Gahn, H. Harrison, M. Harrip, R. Herd, B. Hinton, B. Kinghom, D. Mula, R. Packwood, T. Patterson, D. Perry, T. Snelgar, B. Sundstrom, J. Thompson, R. Thompson, W. Upton, C. Waters, R. Whale and J. Wright.

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traits. 2. Phenotypic and genetic correlations. Anim. Breed. Abstr. 62, 825-853. Meyer, K., 1994. Estimates of direct and maternal correlations among growth traits in Australian beef cattle. Livest. Prod. Sci. 38, 91-105. Mohiuddin, G., 1993. Estimates of genetic and phenotypic parameters of some performance traits in beef cattle. Anim. Breed. Abstr. 61, 495-522. Mrode, R.A., 1988. Selection experiments in beef cattle. Part 2. A review of responses and correlated responses. Anim. Breed. Abstr. 56, 155-167. Nwakalor, L.N., Brinks, J.S., Richardson, G.V., 1986. Selection in Hereford cattle. 1. Selection intensity, generation interval and indexes in retrospect. J. Anim. Sci. 62, 927-936. Parnell, P.F., Morris, CA., 1994. A review of Australian and New Zealand selection experiments for growth and fertility in beef cattle. Proc. 5th World Cong. Genet. Appl. Livest. Prod. 19, 20-27. Schneeberger, M., Tier, B., Hammond, K., 1991. Introducing the thiid generation of B REEDPLAN and GROUPB REEDPLAN. Pm. Aust. Assoc. Anim. Breed. Genet. 9, 194-199. Smith, C., 1984. Rates of genetic change in farm livestock. Res. Dev. Agric. 1, 79-85. Woldehawariate, G., Talamantes, M.A., Petty, R.R., Cartwright, T.C., 1977. A summary of genetic and environmental statistics for growth and conformation characters of young beef cattle. Texas Ag. Expt. Stn. Tech. Bull. 103. (2nd ed.).